Iron base austenitic alloys



Sept. 25, 1956 w. w. DYRKACZ ETAL 2,764,481

IRON BASE AUSTENITIC ALLOYS 4 Sheets-Sheet Effect of Filed Sept. 20, 1954 INVENTORS Wusil W. Dyrkucz Richard K. Plflar and Edward E. Roynoldi BY 5 ATTORNE 2 i by Weight Sept. 25, 1956 w. w. DYRKACZ ET AL 2,764,

IRON BASE AUSTENITIC ALLOYS Filed Sept. 20, 1954 4 Sheets-Sheet 4 INVENTORS Wusil W. Dyrkocz Richard K. Pifler and Edward E. R-eynolds ATTORNEY United States Patent IRON BASE AUSTENITIC ALLOYS Wasil W. Dyrkacz, Newtonville, Richard K. Pitler, Albany, and Edward E. Reynolds, Loudonville, N. Y., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Application September 20, 1954, Serial No. 457,022

7 Claims. (Cl. 75-126) This invention relates to alloys and in particular to iron base austenitic alloys and articles made therefrom which are especially useful at elevated temperatures and in corrosive atmospheres.

Heretofore there have been many alloys known to the art which have been developed for use at elevated temperatures and in corrosive atmospheres, for example, structural and valve components in internal combustion engines. Some of these alloys have been developed to obtain high hardness and strength while sacrificing corrosion and oxidation resistance. Other alloys have been developed to obtain improved corrosion and oxidation resistance characteristics at the expense of the strength and hardness. Still other alloys have been developed in an attemptto combine the properties of high hardness and strength with good corrosion and oxidation resistance together with freedom from phase transformations throughout the temperature range encountered in operation, but such alloys have usually made use of a large amount of highly strategic and expensive alloying elements. Thus there has been a long-standing need in industry for an alloy which would have the combined characteristics of high hardness and strength together with excellent corrosion and oxidation resistance at temperatures up to 1600 F. and higher While at the same time have freedom from phase transformations throughout the temperature range of the intended use, and that such characteristics should be obtained Without the use of expensive and strategic alloying elements, for example, nickel, tungsten, columbium and cobalt.

An object of this invention is to provide an austenitic iron base alloy for use at elevated temperatures in the presence of a highly corrosive atmosphere, which alloy is devoid of strategic alloying elements.

Another object of this invention is to provide an austenitic iron base alloy having high hardness and strength and good resistance to oxidation and corrosion when used in an atmosphere of leaded fuel combustion products.

A further object of this invention is to provide an austenitic iron base alloy for use as structural and valve components having high hardness and strength and very good resistance to oxidation and corrosion together with freedom from phase transformations throughout the operating temperatures of the intended use of this alloy.

2,764,481 Patented Sept. 25, 1956 A more specific object of this invention is to provide an austenitic iron base alloy for use at operating temperatures encountered in gas and steam turbines and internal combustion engines and having manganese and chromium as the major alloying elements, which alloy is characterized by high hardness and strength, very good oxidation and corrosion resistance both at room temperature and throughout the range of operating temperatures of turbines and internal combustion engines together with freedom from phase transformations at temperatures of up to 1600 F. and higher and without the use of strategic alloying elements therein.

Another object of this invention is to provide an austenitic iron base alloy of .45 to .70% carbon, 7.5% to 12% manganese, .25 to .65% silicon, 20% to 23% chromium, .20% to .45 nitrogen and the balance iron with incidental impurities and which will have the characteristics of high hardness and strength together with good oxidation and corrosion resistance when exposed in use to elevated temperatures of up to 1600 F. and higher.

These and other objects will become apparent to one skilled in the art when taken in conjunction with the drawings and the following description in which:

Figure 1 is a graph, the curves of which illustrate the effect of the different alloying elements on the stressrupture properties of the alloys of this invention atan elevated temperature of 1350 F. while under a static tensile stress of 25,000 p. s. i.;

Fig. 2 is a graph, the curves of which illustrate the efiect of the different alloying elements upon the hardness both at room temperature and at 1400" E;

Fig. 3 is a graph, the curves of which illustrate the eifect of the dilferent alloying elements on the resistance to oxidation; and

Figs. 4 through 14 are a series of photomicrographs taken at a magnification of 500 times illustrating the eliect of carbon, manganese, chromium and nitrogen on the phase structure of these alloys.

In its broader aspects the alloy of this invention comprises carbon between about 0.45% and about 0.70%, manganese between about 7.5 and about 12.0%, silicon between about 0.25% and about 0.65%, chromium between about 20.0% and about 23.0%, nitrogen between about 0.20% and about 0.45 and the balance iron with not more than about 1.5% incidental impurities such as phosphorus, sulphur, copper and residual cobalt and nickel such as may occur in standard steel mill melting practice. In some instances it is desirable to include up to 0.15% of either sulphur or selenium or the combination thereof as essential elements in the alloy for improving the machinability of the alloy.

Each of the elements is included in a critical amount to provide a specific function within the alloy. Carbon is needed to impart high strength and hardness as well as to olfset the ferritizing effect of chromium. Manganese performs a function similar to carbon, and is especially useful in maintaining an austenitic structure in this alloy. Silicon is an essential addition to the alloy, its beneficial effect being noted in its effect on the resistance of the alloy to oxidation and corrosion. Chromium is essential within the range stated since it is the principal source of imparting to the alloy the characteristics of resistance to corrosion and oxidation. Nitrogen is included within the limits given to improve the high strength and hardness of the alloy at elevated temperatures and to balance the alloy thus providing a stable austenitic structure.

Reference may be had to Table I giving the chemical composition of the alloy of this invention illustrating both the general range and the optimum range, it being noted that where the balance is listed as iron, it is understood that such balance includes incidental impurities, with or without sulphur and/ or selenium as essential elements in the amounts given hereinbefore.

TABLE I jCh enticizl cbrhfibsiiioW-pfcent by weight Elenint General Optimum Range Range (The aillbybfjthis invention may be produced by any of the wen-know steel mill practices, for example, by

electric furnacemelting. :Preferably a charge consisting of scrap ina ng'anese, chromium and slag forming mateal's is arcinlted in an electric furnace to provide a rriolten (metal bath. The elements manganese, silicon and chromium may be present in the charge in any suit- .manner. Whe i 1 th,e molten metalhassolidified, it may e r ggae @br 11 1 rolled irrtQ/a'Smi-finished 6011 4 651, for example, bar,"billet, rod, slab, plate, sheet or strip. Ifromthe semi-finished mill product, the alloy of this inforvalvecolm'poherits for anint ern al combustion engine or into structural components. The finrshed product is then heat tre ated t be more fully described, to produce the o mproperties in this alloy. 7 I

; Since thealloy in itsp'referred form is completely auste: tic, it'must be age hardenedin some applications fto provide the requ site hardness and strength. The prej edi heat treatment employe'dconsists of asolut ion "heat treatment atatemperature in the range between lQbQYF. and 2300 F. for a 'time period ranging between 15 minutes and 8 hours. After the alloy has been held at temperature for a sufiicient period of time to produce a completely austenitic structure, it cooled to roomtemperature by quenching in a liquid medium.

It will be appreciated, however, that if extremely thin sections are employed, air cooling from the solution he'at t reating temperature may suffice. The solution heat treatment may be followed by anaging treatment at a 'temperaturein the range between 1200 F. audit 590 F. for a period of time ranging between 15 minutes and 48 hours. The agingltreatment has theef fect of increasing the hardness at both the room temperature and elevated a =9 It will be appreciated that the heat treatment of the alloys as hereinbefore outlined will depend upon their "from which itis teemed into iugots in a conventional 7' button .is 1 1116 1 1 0 final shape, for example, valve I I 4 end use. This can best be illustrated by considering the properties required for representative uses. For example, if the alloy of this invention is to be used as a structural component, for example, a turbine vane or bucket, the primary consideration is given to creep and rupture strengths. Since the alloy of this invention is an age-hardening alloy, it can readily be appreciated that for use as such a structural component, the alloy may be used in its solution treated condition where the creep and rupture strengths are the highest. On the other hand, Where thermal shock and resistance to wear by erosion and abrasion are of paramount importance, for example, in valves and valve components, the alloy is preferred in its aged condition. It is also apparent that there is a range over which the ductility, elevated temperature strength, hardness and degree of structural stability can be maintained by a selection and/or variation in the heat treatment within the ranges set forth hereinbefore.

In order to show the efiects of each of the elements, both within and outside of the general range, a number of alloys having the chemical compositions given in Table II were made and tested.

TABLE II Chemical COMPOSIHOH (weight percent) alloys made'mzd tested Alloy No. C Si Mn Cr N Fe 0.57 10.26 9.67 .19.12 0.252 Ba]. .057 0. 39 9. 52 24.42 0.- 262 Ba]. 0. 56 0. 10.01 18. 59 0266 B81. 0.56 0.29 9.94 19.56 0.260 'Bal. ,0. 55 0. 25. 10.01 .20. 68 0. 268 Ba]. 0.59 0.25 10.12 21.49 0.272 'Ba]. 0. 56 '0132: 110. 01 22.73 0. 241 Ba]. 0. 59 0. 31 10.12 23.85 ,0. 260 E81. 0. 57 0.11 8. 80 '22. 05 0. 394 E91. 0. 59 0. 24 8. 51. 21.85 0.349 Bal. 0. 60 0. 54 8. 40 21.97 0.368 Bal. 0.60 0.92 8.56 21.97 0.374 'Bal. ,0. 48 .044 8. 20 21.43 0.324 Bal. 0. 61 0.97 8. 29 22. 22 0. 322 Ba]. 0. 20 0.30 8. 00 21.95 0. 343 B6].

. 0.45 0. 28 8. 00 21.85 0. 368 Bal. 0,83 0.30 8.00 22.01 0.345 .Bal. 1. 01 0. 34 7. 96 21. 71 0. 347 Hal. 0, 53 0. 40 5.09, 22.52 0. 360 Bal. 0. 59 0. 43 6. 30, 22.02 0. 374 B01. 059' 0.41 7.96 22.34 0.358 Bal. 0.60 0.43 8.03 21.77 0.378 Bal. 0. 63 0. '41 10. 29 22.28 0. 356 B01. 0. 60 0.42 12. 80 22.18 0.356 Bal. 0.46; 0. 32* 14.42 22.43 0.347 ,Bal. 0.50 0. 32 8. 40 22.25 0. 443 Hal. .0. 68 0; 34 8. 63 22.76 0. 584 E81. 0. 57 0.39 8; 83 22. 09 0. 030 1381. 0. 62 0. 39 8. 66 22.33 0. 169 1381. 0; 63 0.38 8.88 22.08 0. 269 B211.

p d l pon 'theil' 'stra in'rate, the stress-rupture test was employed to give a more representative view of the "strength of the alloysbf thisinve'ntion than can be had from the short time tensiletest. The stress-rupture test is 'usuallyp'er'fornied at stress levels below the 'yield 'st'rengthand 'as'such these tests illustrate the b ehavior of the 'alloy under conditions similar to those which are encountered in the intended use of the alloy, for example, as structural components" 'or"as valve components of an internal combustionengine.

Reference mayibe'had' to Table'III illustrating the effect of the differentalloyingeleinents on the stress-rupture properties .when measured at elevated temperatures. The alloys of Table II have been rearrangedin Tables III, IV, V and ,VI to be referred tohei'einaften'to illustrate the effect of the elements'carb'on, manganese, silicon, chromium and'nitrogen on the different proper-ties, stressrupture, hardness,'oxidation and corros'ion resistance, respectively. In the stress-rupture test each of the alloys was subjected to a static tensile stress at a given temperature as set forth in the heading of Table III. The results were recorded in terms of the time to produce rupture and the time to produce 1% total strain.

TABLE III Time (Hours) to- Per- Per- Per- Per- Per- Alloy No. cent cent cent; cent cent Si Mn Gr Rupture 1% Total Strain 0.41 10. 29 22. 28 0.36 237 35 0. 42 12. 80 22. 18 0.36 138 21 0. 32 14. 42 22.43 0.35 100 11 O. 11 8. 80 22.05 0.39 274 25 0.28 8. 00 21.85 0.37 254 25 0.43 8. 63 21. 77 0.38 276 18. 8 0. 92 8. 56 21.97 0.37 156 13. 0.61 0.97 8.29 22.22 0.32 59 6 0. 56 0.30 10. 01 18.59 0.27 318 35 0. 57 0. 26 9. 67 19. 12 0. 25 296 24 0.56 0.29 9. 94 19. 56 0. 26 346 19 0. 55 0.25 10.01 20. 68 0. 27 263 16 0. 59 0.25 10. 12 21.49 0. 27 253 16 0.56 0.32 10.01 22.73 0.24 299 0.59 0. 31 10.12 23.85 0.26 113 6. 5 0. 57 0.39 9. 52 24. 42 0.26 83 0.3 O. 57 0.39 8. 83 22. 09 0. 03 2. 5 0. 1 0. 62 0. 39 8. 66 22. 33 0. 17 92 3. 8 0. 63 O. 38 8. 88 22. 08 0. 27 186 26. 8 0. 60 0. 43 8. 63 21. 77 0. 38 276 18. 8 0. 59 0.32 8. 40 22. 25 0.44 201 15 0.63 0.34 8. 63 22.76 0.58 124 15 By inspection of the results recorded in Table III, for the first five alloys listed therein, it can be seen that carbon substantially increases the stress-rupture properties up to about 0.60% carbon. By comparing alloy Nos. G-241, G-242, G-271, G-243 and G-244, it can be seen that when the carbon content is increased much beyond 0.70%, the time to produce rupture and to. produce 1% total strain sharply drops as illustrated by comparing alloys G-271 and G-243. In addition, carbon contents beyond the upper limit of 0.70% make this alloy extremely difficult to machine as will be more fully described with respect to hardness. Below about 0.45% carbon, the stress-rupture properties are very low. The effect of varying the carbon content on the time to rupture is particularly illustrated by reference to curve 10 of Fig. 1 which clearly illustrates that the carbon content is critical within the range stated in Table I and as is evidenced by the peak in the curve of Fig. 1. Since carbon is an austenite forming element, it is desirable to maintain at least 0.45% carbon in this alloy for the reasons to be more fully described with respect to Figs. 4 and 5, the optimum carbon content being about 0.60% which corresponds to the peak portion of the carbon curve of Fig. l.

Manganese behaves in a manner similar to carbon in that stress-rupture properties are greatly increased when the manganese content is maintained between 7.5% and 12.0%. This can be seen by comparing the results recorded in Table I for alloys Nos. G-268, (3-269, (3-271, G-272, G-273 and G-274 wherein the manganese content ranges from 5.09% to 14.42%, and by curve 12 of Fig. 1 which illustrates the effect of varying the manganese on the time to rupture. While the results recorded in Table III and Fig. 1 would indicate that 6.5% manganese could be tolerated for the lower limit without adversely aifecting the strength, since manganese is also an austenite forming element, it is preferred that a minimum of about 7.5% be maintained in order .to offset the ferritizing effect of chromium and silicon and thereby maintain the austenitic structure of the alloy. When the manganese content is increased'above 12.0%, the stress-rupture properties drop sharply as is evident in Table III in the time to produce rupture and more clearly shown by curve 12 of Fig. 1. While the 1% total strain data is somewhat erratic, it shows the same general trend. It has been found that increasing the manganese content from 10.0% to the upper limit of 12.0% is advantageous only if the other elements are so proportioned as to require such quantities of manganese to obtain a substantially balanced austenitic structure in the alloy.

Silicon is an essential element in the alloy of this invention. While silicon has a great eifect on the stressrupture properties as illustrated by curve 14 of Fig. 1 and the results obtained on alloy Nos. G-122, G-242, G-271, G-l93 and G-207 as grouped in Table III, it is quite evident that increasing the silicon up to about 0.65% does not produce any substantial change in the stress-rupture properties. However, when the silicon content is increased above 0.65 the stress-rupture properties drop rapidly. This is clearly illustrated by the results given for the above identified alloys both in the time to produce rupture and in the 1% total strain time. A more beneficial effect is realized from silicon in relation to oxidation and corrosion resistance as will hereinafter be more fully described.

By inspection of Table III, the eifect of increasing chromium from 18.59% to 24.42% can be seen by comparing 'the results obtained on alloy Nos. G-llO, A-837, G-lll, G-112, G-113, G-114, G- and G-41. It is quite evident that increasing the chromium content beyond 23.0% seriously affects the stress-rupture properties of this alloy. This is more amply illustrated by reference to curve 16 of Fig. 1. The effect of increasing chromium is reflected both in the time to produce rupture and in time to produce 1% total strain. While there is some decrease in the stress-rupture properties between 20.0% and 23.0% chromium, the decrease is insignificant when balanced against the beneficial efiect of chromium on the resistance to oxidation and corrosion to be more fully described hereinafter. The lower limit of the chromium range as set forth in Table I is governed by the resistance to oxidation and corrosion as will be more fully described. However, it is quite evident from curve 16 of Fig. 1 that .the upper limit of chromium must be limited to about 23.0%. The slope of the chromium curve for chromium contents above 23.0% is so steep that it clearly illustrates that slight increases beyond 23.0% produce a severe drop in the time to produce rupture in these alloys.

Nitrogen behaves in a manner somewhat similar to carbon and manganese with regard to the stress-rupture properties, in that there is a definite peak in curve 18 of Fig. 1. Curve 18 of Fig. 1 illustrates the eflect of nitrogen on the stress-rupture properties of these alloys. By comparing the results obtained on alloy Nos. G-277, G-278, G-279, G-271, G-275 and G-276 of Table III, it can be seen that with a nitrogen content below 0.20%, the stress-rupture properties are quite low. When the nitrogen content is increased beyond 0.45%, for example, as in alloy G-276 which contains about 0.58% nitrogen, the time to produce rupture falls ofi considerably when compared with alloy G-275 which contains about 0.44% nitrogen. The nitrogen content should therefore be limited to the range 0.20% to 0.45% as set forth in Table I. The nitrogen also cooperates with the carbon and manganese to offset the ferritizing effect of the chromium and silicon and thereby renders the alloy austenitic. Other considerations for limiting the nitrogen content will be more fully discussed with respect to the results given in Table V.

F rom Table III and as more clearly illustrated by curves 10, 12 and 18 of Fig. 1, it can be seen that carbon, manganese and nitrogen, within the limits as set forth in Table I, greatly increase the stress-rupture properties of these alloys when measured at elevated temperatures, by increasing both the time to produce rupture as well as the time to produce 1% total strain. While chromium and 7 silicon are essentially used for their advantageous effect on the corrosion and oxidation resistance, they do not significantly adversely affect the stress-rupture properties as illustrated by curves 16 and 14, respectively, of Fig. 1 when maintained within the limits as set forth in Table I.

As was stated previously, the alloy of this invention is especially adapted for use as valves and valve components in an internal combustion engine. In such an application it is of special importance that the alloys of this invention possess sufficient hardness to withstand the wear by erosion and abrasion which is occasioned by the passage of air and hot gases which contain combustion products of leaded fuels. Valves and valve components are also subjected to heavy wear by their movement within the valve guides and by operation of the valve tappet upon the valve tip when the valve is actuated. Since the valves and valve components must function over a wide temperature range between room temperature and the operating temperature of the engine, it is apparent that the alloy must retain its hardness at the elevated operating temperatures.

Reference may be had to Table IV which illustrates the hardness properties of some of the alloys listed in Table II both at room temperature and at 1400 F. and both in the quenched and aged conditons. The hardness test of these alloys was taken at 1400 F. and consisted of heating the alloy to 1400 F. and thereafter lowering a Brinnell hardness penetrator into the furnace Where the indentation was made in accordance with the standard method of testing at room temperature. The tested alloy was then removed from the furnace, cooled, and the diameter of the indentation measured. The Brinnell hardness number was then calculated according to the standard equation for determining the Brinnell hardness number. The alloys of Table IV have been rearranged in a manner similar to the arrangement of Table III to show the effect of each of the alloying elements on the hardness of the alloys.

TABLE IV Hardness properties Brinell Hardness Number State State: 2,150 F Alloy Per- Pcr- Per- Per- Per- 2,150 F. Water No. cent cent cent cent cent Water Quench-l- C Si Mn Cr N Quench 1,400 F.-

16 hrs. A. 0.

Room 1,400 Room 1,400

Temp. F. Temp. F.

Alloy Nos. G-241, G-242, G-271, 6-243 and G-244 of Table IV illustrate the effect of increasing the carbon content from 0.20% to 1.01% on the hardness of the alloy. Increasing carbon contents produce increased hardness in this alloy both at room temperature and at 1400 F. and in both the quenched and aged conditions. The increase in hardness which accompanies increasing carbon contents is graphically illustrated by reference to Fig. 2 in which curves 20 and 22 show the effect of carbon on the solution treated hardness at room temperature and at 1400 F., respectively, and curves 24 and 26 "show the effect of carbon on the hardness in the aged condition at room temperature and 1400" F., respectively. Thus, by comparing the results shown in Figs. 1 and 2, while the strength of these alloys attains a peak at about 0.60% carbon and any increase in carbon content over 0.70% effects a sharp drop in the time to produce rupture as illustrated by curve 10 of Fig. 1, increasing carbon contents over 0.70% does not produce a corresponding decrease in hardness, as illustrated by curves 2022 and 2F26 of Fig. 2. It should not be concluded that since increasing the carbon'content produces corresponding increases in hardness, the strength at elevated temperatures should also be correspondingly increased. As was pointed out hereinbefore, metals deform in different manners at elevated temperatures depending upon the strain rate; hence, there is no correlation between the effect of each of the elements on the hardness as compared to the rupture strength, since there is an entirely different state and rate of stress applied to the alloys in each of the tests for hardness as compared to strength. Also, as referred to in the discussion of the data in Table III, increased carbon contents above 0.70% made machining of this alloy quite difficult. This is especially true in the aged condition as illustrated by the high hardness of more than 360 BHN of alloy N0. G-244. Thus, from Tables III and IV and as illustrated in Figs. 1 and 2, carbon greatly contributes to both the strength and hardness of the alloy of this invention.

The effect of increasing the manganese content from 5.09% to 14.42% on the hardness of the alloy is'illustrated by comparing the results given for alloy Nos. G- 268, 6-269, G-270, G-192, G-271, G-272, G-273 and 6-274 in Table IV. Such test results clearly demonstrate that manganese has little effect on the hardness of this alloy. This is more clearly illustrated by reference to curves 28 and 30, and 32 and 34 of Fig. 2, which show the effect of manganese on the hardness of the alloy in the solution treated condition and aged condition, respectively, the curves 28 and 32 showing the results at room temperature and the curves 30 and 34 showing the results obtained at 1400" F. In Table III, the stressrupture properties were increased and attained their optimum value when the manganese content was maintained within the range 7.5% to 12.0% as given in Table I. However, there seems to be little effect upon the hardness of these alloys at 1400 F. with increasing manganese content even up to 14.42%; while at room temperature, high manganese contents, that is, over 12.0%, decrease the hardness slightly. This again shows that there is no correlation between the hardness and rupture strength exhibited by each of the alloying elements. It is noteworthy to point out that the effect of each of the alloying elements, for example, manganese, is substantially similar on the hardness as measured in the solution treated or aged condition Whether measured at room temperature or at 1400 F.

The effect of silicon Within the range between 0.11% and 0.97% on the hardness is illustrated by comparing the results obtained on alloy Nos. G-122, G-191, G-27l, 6-192, G-193, 6-207. The results of the hardness te'stcan be more clearly seen by reference to'curve's 36- 38 and 40-42 of'Fig. '2. It is'seen that the curves 36 and 38 based on results obtained on the solution treated alloys show a very slight increase in hardness with increasing silicon contents. However, the aged alloys as shown by curves 40 and 42 illustrate no particular trend at room temperature or at 1400 F. Thus from Table IV and Fig. 2 it can be seen that silicon has little efiect upon the hardness of these alloys. Thus from the standpoint of strength and hardness as illustrated in the curve 14 of Fig. 1 and curves 36-38 and 4042 of Fig. 2, respectively, it is quite evident that the silicon content must be limited to about 0.65% maximum since silicon contents in excess of this value have a detrimental effect on strength.

Chromium behaves in a manner somewhat similar to silicon with respect to its elfect on the hardness of the alloys of this invention. Thus increasing chromium contents from 18.59% to 24.42% produces no significant effect on the hardness of these alloys. This can be seen by comparing the results obtained on the hardness tests performed on alloy Nos. Gl10, G-lll, G112, G-113, Gl 14, G-115 and G41. This effect of chromium is also illustrated by reference to curves 44-46 and 48-50 of Fig. 2 which show the effect of chromium on the hardness of these alloys in the solution treated condition and aged condition, respectively, when tested at room temperature and at 1400 F. While in some austenitic alloys it has been found that chromium in solid solution contributes to the hardness and strength even though it is a strong ferritizing element, there is no significant increase in the hardness or rupture strength on the austenitic alloy of this invention containing between 20.0% and 23.0% chromium as evidenced by reference to curves 44-46 and 4850 of Fig. 2, and curve 16 of Fig. 1, respectively. When the chromium content is maintained Within the range between 20.0% and 23.0% as set forth in Table I, it will have a beneficial effect on the oxidation and corrosion resistance as will be more fully described hereinafter.

By increasing the nitrogen content from 0.03% to 0.58% as illustrated by reference to Table IV and the results obtained from tests on alloy Nos. G-277, G-278, G-279, G271, G-275 and G276, a definite increase in the hardness both at room temperature and at 1400 F. is observed. The increase in hardness is especially noticeable when the nitrogen content is increased from 0.30% up to 0.20% as illustrated by comparing alloy Nos. G-277 with G-278. The increase in hardness produced by nitrogen is more clearly illustrated by reference to curves 52-54 and 5658 of Fig. 2. While increasing the nitrogen content produces a corresponding increase in the hardness, it is desirable to maintain such nitrogen content at not more than 0.45% because of the adverse eifect of higher concentrations of nitrogen on the rupture strength as shown by curve 18 of Fig. 1 and because of the detrimental effect on the oxidation resistance as will be more fully described hereinafter. From the results recorded in Table IV and the curves of Fig. 2, it can be seen that carbon and nitrogen are the principal elements which control hardness characteristics of the alloys of this invention.

As was stated hereinbefore, the alloy of this invention is especially adapted for use as structural components and as valves and valve components in an internal combustion engine. Since these alloys are intended for operation at elevated temperatures, it is extremely important that they possess a good resistance to oxidation at such operating temperatures. Reference maybe had to Table V showing results of tests conducted on the alloys of this invention when tested for oxidation resistance by exposing the alloys of this invention :for time periods of 100 hours at 1700 F. The alloys of Table V have been regrouped to show the effect of each of the alloying elements on the oxidation resistance. Specimens of the 10 alloys in the form of a cylinder .500" in diameter and .500" long were weighed, exposed and then descaled electrolytically in a molten salt'bath and reweighed. The

resistance to oxidation is reported as the percent weight loss calculated on the basis of the original weight.

TABLE V Percent Percent Percent Percent Percent Percent Alloy No. C Si Mn Cr N W Loss 0. 53 0. 40 5. 09 22. 52 0. 36 O. 88 0. 59 0.43 6. 30 22. 02 0.37 0. 90 0. 59 0.41 7. 96 22. 34 0.36 0. 93 0. 63 0. 41 10. 29 22. 28 0. 36 0. 99 0. 60 0.42 12. 22. 18 0.36 0. 94 0.46 0.32 14. 42 22.43 0. 35 0.87 0. 57 0.11 8.80 22.05 0.39 1. 23 0.45 0. 28 8.00 21. 0.37 0.55 0. 61 0.97 8. 29 22. 22 0.32 0. 25 0. 56 0.30 10.01 18. 59 0. 27 3. 59 0. 56 r 0.29 9.94 19. 56 0. 26 O. 80 0. 55 0. 25 10. 01 20. 68 0. 27 0. 04 0. 59 0. 25 10. 12 21. 49 0. 27 0. 57 0. 56 0. 32 10. 01 22. 73 0. 24 0. 51 0. 59 0. 31 10. 12 23. 85 0. 26 0.47 0. 57 0.39 8. 83 22. 09 0.03 0. 51 0. 62 0. 39 8. 66 22. 33 0. 17 0. (i1 0. 63 0. 38 8. 88 22. 08 0. 27 0. 78 0.59 0.32 8. 40 22.25 0. 44 1. 46 0.20 0. 30 8. 00 21.95 0.34 0.31 0. 45 0. 2B 8. 00 21. 85 0. 37 0. 55 0.83 0.30 8. 00 22.01 0.35 0. 67 1. 01 0. 34 7. 96 21. 71 0. 35 0. 68

Referring particularly to the results recorded in Table V of tests obtained on zalloy Nos. G-268, G-269, G-270, G-272, G-273 and G-274, and as more clearly shown by curve 60 of Fig. 3, it canreadily be seen that manganese has little efiect on the resistance of these alloys to oxidation. By comparing alloy G-269 with alloy G-273, it can beseen that by doubling the manganese content from about 6.0% to about 12.0% produces no significant dilference in the weight loss. Thus as was hereinbefore stated, the primary function of manganese is to provide elevated temperature strength and to cooperate with the carbon and nitrogen to provide the alloy with its austenitic structure.

As was stated hereinbefore, silicon has a pronounced effect upon the resistanc to oxidation and as such, silicon is considered an essential element in the alloy of this invention. By comparing the results obtained by testing alloy Nos. G-122, G-242 and G-207, it can be seen that by increasing the silicon content from about 0.10% to 1.0% has the effect of greatly decreasing the weight loss. This is more clearly illustrated by reference to curve 61 of Fig. 3. Comparing alloy G-122 with alloy G-242, an increase in the silicon content of from 0.11% to 0.28% produces a decrease in the weight loss of about 60%. Thus, while silicon within the range disclosed in Table I has a slight effect on the rupture strength and hardness at elevated temperatures as was hereinbefore described with respect to Tables III and IV, curve 14 of Fig. 1, and curves 36-38 and 40-42 of Fig. 2, its outstanding effect upon the oxidation resistance far surpasses any slight decrease in elevated temperature strength and hardness.

The greatest improvement in the oxidation resistance is produced by the chromium content. Comparing the test results from alloy Nos. G-l10, G-lll, G-112, G.-1 13, G-l14 and G-115, increasing the chromium content from 18.59% to 23.85% produces an outstanding decrease in the percent Weight loss in the alloy of this invention. As can be seen by comparing alloy G-112 1'1 with alloy G-115, the optimum resistance to oxidation occurs within the general chromium range as set forth in Table -I. Referring to curve 62 of Fig. 3, it is evident that an increase in the chromium content beyond 23.0% does not produce a correspondingly greater increase in oxidation resistance such as is occasioned by increasing the chromium content, for example, from about 19% to about 20%. Therefore, since chromium is a strong ferritizing element and it is desirable to maintain the alloy of this invention au-stenitic, and as was stated previously with respect to the decrease in elevated temperature strength which accompanies chromium contents in excess of 23.0%, it ispreferred to limit the chromium content to the range 20.0% to 23.0% as set forth in Table I.

By comparing the test results recorded in Table V on alloy Nos. G-277, G-278, 6-279, and 6-275 and the curve 64 of Fig. 3, the effect of nitrogen within the range between 0.03% and 0.44% on the oxidation resistance can readily beseen. By increasing the nitrogen by about 0.17% as in the case of alloy Nos. 6-279 and G-275, a two-fold increase in the weight loss is obtained. While this detrimental effect may seem to be great, the increase in strength and hardness at elevated temperatures occasioned by the use of nitrogen together with its ability to cooperate with the carbon and manganese to offset the ferritizing effect of chromium and silicon and thereby maintain the austenitic structure of the alloy justifies the use in this alloy of a nitrogen content in the range 0.20% to 0.45 as stated in Table I.

The effect of carbon on the oxidation resistance is evident from the results recorded for alloy Nos. 6-241, 6-242, G-243 and G-244 in Table V and as illustrated graphically by curve 66 of Fig. 3. As recorded, increasing the carbon content of the alloys decreases the oxidation resistance, but when maintained within the preferred range of this invention, has substantially no significant detrimental effect thereon.

Thus from the results recorded in Table V and curves 61 and 62 of Fig. 3, it is readily seen that the elements silicon and chromium provide the necessary resistance to oxidation within the alloys of this invention. Manganese has little effect upon the oxidation resistance. While carbon and nitrogen have a somewhat detrimental effect on the oxidation resistance, the use of such elements is justified by their beneficial effects on the hardness and rupture strength.

Since the alloys of this invention are especially adapted for use in internal combustion engines as valves and valve components, it is extremely important that the alloys possess a resistance to corrosion, especially in an atmosphere composed of the products of combustion of leaded fuels. The principal test used to evaluate the corrosion resistance of these alloys is the lead oxide cone test in which a small cube of the alloy to be tested is covered on its upper surface with a cone of lead oxide powder of a predetermined mass. The sample is then placed in a furnace, held at a temperature for two hours and thereafter removed. The temperature at which successive samples were tested was determined by the extent to which the corrosive attack had progressed on the preceding samples thereby determining the temperature at which the corrosive attack begins and the temperature at which the corrosive attack -is completed. These tests were run at temperatures in the range between 1350 F. and 1550 F. It is believed that this test simulates corrosive conditions at temperatures such as are encountered during operation of an internal combustion engine. In order to evaluate the corrosion resistance, reference may be had to Table VI illustrating the effect of silicon and chromium upon the resistance to corrosion. The values recorded are the temperatures :at which the corrosive attack of the lead oxide begins and the temperature at which the corrosive attack is completed.

12 TABLE VI C orrosfon resistance-Lead oxide cone test Temperature (F) Por- Per- Per- Ier- Perof Attac Alloy N0. 'cent 0 cont; Si tislnt cent Cr cent N Start Complete (It-122m. 0. 57 0. 11 8.80 22. 05 0. 39 1, 385 1, 425 Gl9l t). 59 0. 24 8. 51 21. 0. 35 1, 425 1, 435 G-271 0. 60 0. 43 8. 63 21. 77 0.38 1, 430 1, 440 G206 0. 48 0. 44 8. 20 21. 43 0. 32 1 427 1, 440 (Bl-192."v 0. 60 0. 54 8. 40 21. 97 O. 37 1, 435 1, 445 G-193 0. 60 0. 92 s. 56 21. 97 0. 37 1, 435 1, 445 G207 0. 61 l). 97 8. 29 22. 22 0. 32 1, 438 1, 442 (Ii-110.." 0. 56 0. 3O 10. 01 18. 59 0. 27 1, 365 1, 425 11-837.. 0. 57 0. 26 9. 67 19. 12 0. 25 1, 375 I 415 G1ll r 0. 56 0. 29 9. 94 19. 56 0. 26 1,365 1, 420 G112 0. 55 0. 25 10. 01 20. 68 0. 27 1, 434 1, 440 G1l5 0. 59 0. 31 10. 12 23. 85 0. 26 1, 466 1, 472

By comparing the results recorded in Table VI on alloy Nos. 6-122, 6-191, 6-271, G-206, 6-192, 6-193 and G-207, it can be seen that increasing the silicon content from 0.11% to 0.97% produces an increase in the temperature at which the corrosive attack starts and also an increase in the temperature at which the corrosive attack is completed. Thus by increasing the silicon content in the range given, an increase is obtained in the temperature at which the corrosive attack begins. For example, by increasing the silicon from about 0.10% to about 0.55% as in alloy Nos. G-122 and G-l92, an increase of about 50 F. is obtained in the temperature at which the corrosive attack begins. A similar increase, but of-different magnitude, is noted in the temperature at which the attack is completed. Thus, silicon within the range given in Table I is quite advantageous both to oxidation resistance as was stated hereinbefore with respect to Table V, and t0 the corrosion resistance as is evident in Table VI.

Referring to the test results obtained on alloy Nos. G-ll'O, A-837, G-lll, G-l12 and G-115, it can be seen that increasing the chromium content from 18.59% to 23.85% produces a definite increase in the temperature at which the attack begins and is completed.

In the foregoing description, emphasis has been placed on the austenitic structure of the alloy of this invention. It has thus become apparent from the discussion of Tables III through VI that the alloy of this invention must be maintained within the ranges of the alloying elements as set forth in Table I in order to have the proper balance between strength, hardness and resistance to oxidation and corrosion. It is also necessary for the alloy of this invention to be maintained within the limits as set forth in Table I in order that the alloy be predominantly austenitic in structure. In order to more clearly illustrate the effect of carbon, manganese, chromium and nitrogen on the austenitic structure of these alloys, reference may be had to the photomicrographs of Figs. 4 through 14 showing the effect of these elements on the structure of these alloys. Each of the photomicrographs was taken at a magnification of 500 times on the alloy after heating for one hour at 2150" F. followed by a water quench. Each of the alloys of Figs. 4 through 14 has its composition recorded in Table II.

Fig. 4 shows the structure of alloy No. G-241 in which all of the alloying elements except carbon are within the general range as set forth in Table I. While the minimum carbon content from Table I is 0.45% carbon, alloy No. G-24l with a carbon content of 0.20% is substantially below this limit. This alloy as seen in Fig. 4 has a duplex structure consisting of a matrix of austenite 90, with substantial amounts of ferrite 92. Thus, when the carbon content is below the minimum of 0.45% carbon, the alloy is far from being predominantly austenitic in structure. However, upon increasing the carbon content to about 0.45 carbon, the ferrite 92 as shown in Fig. 4 disappears and the alloy is predominantly austenitic, as shown by the photomicrograph of alloy No. G-242 in Fig. 5. Alloy No. G-242 has substantially the same composition of alloy No. G-241 except the carbon content of 0.45% is within the range set forth in Table I. Thus, by increasing the carbon content from 0.20% to 0.45%, all of the ferrite 92 in Fig. 4 has disappeared as shown in Fig. 5 and the alloy is predominantly austenitic in structure. By increasing the carbon content to 0.60% carbon and 0.83% carbon, as in alloy Nos. G-271 and G-243, respectively, the balance of the alloyingelements being substantially the same as in alloy Nos. 6-241 and G-242, an increasein the amount of carbides 94 contained in these alloys is obtained as illustrated in Figs. 6 and 7, respectively. Thus, while alloy No. G-271 shows a slight increase in the amount of carbides 94 when the carbon content is increased from 0.45 carbon to 0.60% carbon, when the carbon is increased up to 0.83%, as in the case of alloy 6-243, substantial amounts of carbide 94 are present as shown in Fig. 7. It is for this reason that carbon contents above the maximum specified in Table I makes this alloy difficult to machine.

The effect of manganese in the range between 8.63% and 14.42% on the austenitic structure of these alloys is illustrated by reference to the photomicrographs of alloy Nos. G-271, G-272 and G-274 forming Figs. 6, 8 and 9, respectively, each of the alloys having substantially the same composition with the exception of the manganese content. The structure of alloy No. G-271 containing 8.63% manganese and shown in Fig. 6 is austenite 90 with some carbides 94. Increasing the manganese content to 10.29% as shown in Fig. 8 of alloy No. G-272 does not substantially change the structure of this alloy from that shown in Fig. 6. However, when the manganese is increased above the upper limit of 12.0% as in alloy No. 6-274 of Fig. 9 which contains about 14.42% manganese, the predominantly austenitic structure of Fig. 8 is changed by the formation of an unidentified phase 96. It is evident that while the alloy of Fig. 9 contains a matrix of austenite 90, substantial amounts of an unidentified phase 96 have also appeared. It is therefore apparent that the manganese content must be maintained in the range 7.5% to 12.0% as set forth in Table I in order for the alloy to be predominantly austenitic in structure.

In order to show the efieot of chromium on the structure of the alloy of this invention, reference may be had to Figs. 10 and 11 which are photomicrographs of alloy Nos. G-114 and G-l15, respectively, having substantially the same composition except for chromium contents of 22.73% and 23.85%, respectively. Alloy No. G-114 of Fig. 10 has a structure of austenite 90 containing carbides 94. It can readily be seen that the structure of this alloy is predominantly austenitic. However, by increasing the chromium content above the maximum of 23.0% as illustrated by the photomicrograph of Fig. 11 of alloy No. G-115 and which contains 23.85% chromium, it can be seen that a duplex structure consisting of a matrix of austenite 90 containing substantial amounts of ferrite 92 and some carbide 94 is formed. It is apparent therefore that the chromium content must be maintained below about 23.0% maximum in order for the structure of this alloy to be predominantly austenitic.

Nitrogen is a well known austenite forming element. Its effect on the structure of this alloy in the range between 0.03% and 0.27% nitrogen is shown by reference to Figs. 12, 13 and 14 which are photomicrographs of alloy Nos. G-277, G-278 and G-27 9, respectively. Each of these alloys has substantially the same composition except for the nitrogen contents which are 0.03%, 0.17% and 0.27% nitrogen, respectively. Alloy No. 6-277 of Fig. 12 containing 0.03% nitrogen which is below the lower limit of the nitrogen content given hereinbefore is found to exhibit the duplex structure of ferrite 92 and carbide 94 in a matrix of austenite 90. By comparing the photomicrograph of Fig. 12 with that of Fig. 13 for alloy No. G-278, it is seen that an increase in the nitrogen content from 0.03% to 0.17% has resulted in a substantial decrease in the amount of ferrite 92 present within the alloy. By funther increasing the nitrogen up to 0.27% as in alloy No. G-279 forming the basis of the photomicrognaph of Fig. 14, the ferrite 92 found in the alloys of Figs. 12 and 13 has been eliminated and the alloy becomes predominantly austenitic with some carbide 94 present. It is therefore apparent that the nitrogen content of the alloys of this invention must be at least 0.20% nitrogen. From the foregoing discussion of the microstructure of the alloys illustrated in Figs. 4 through 14, it is apparent that in addition to the reasons given with respect to the physical properties of these alloys, the alloying elements must be maintained within the range as set forth in Table I in order for these alloys to have a predominantly austenitic structure.

From the foregoing descniption, it is apparent that the alloy of this invention is quite useful. Further, the alloys of this invention are readily weldable which characteristic renders them highly desirable for structural usage. No special skills or equipment are required in its manufacture or heat treatment or in its fabrication. It is especially useful as any article of manufacture, for example, turbine buckets, vanes, rotors, tail cone, afterburner and regenerator parts and as structural components in turbines and as valves and valve components in internal combustion engines.

We claim:

1. An :age hardenab-le austenitic alloy consisting of. from 0.55% to 0.65% carbon, from 7.5% to 10.0% manganese, 0.45% to 0.60% silicon, from 21.0% to 22.5% chromium, from 0.30% to 0.40% nitrogen, and the balance iron with incidental impurities.

2. An age hardenable austenitic iron base alloy for use as structural and valve components consisting of, about 0.60% carbon, about 8.4% manganese, about 0.54% silicon, about 21.97% chromium, about 0.37% nitrogen. and the balance iron with incidental impurities.

3. An age hardenable austenitic iron base valve having high strength and hardness and good resistance to corrosion in the presence of leaded fuel combustion products at valve operating temperatures and containing as essen tial alloying elements about 0.55% to 0.65% carbon. about 7.5% to 10.0% manganese, about 0.45% to 0.60% silicon, about 21.0% to 22.5% chromium, about 0.30% to 0.40% nitrogen, and the balance iron with incidental impurities.

4. An age hardenable austenitic iron base valve having high strength and hardness and good corrosion resistance in the presence of leaded fuel combustion products at valve operating temperatures consisting of, about 0.60% carbon, about 8.4% manganese, about 0.54% silicon, about 21.97% chromium, about 0.37% nitrogen, and the balance iron with incidental impurities.

5. An age hardenable austenitic alloy consisting essentially of, from 0.55% to 0.65% carbon, from 7.5% to 10.0% manganese, from 0.45% to 0.60% silicon, from 21.0% to 22.5% chromium, from 0.30% to 0.40% nitrogen, up to 0.15% of at least one of the machinability improving elements selected from the group consisting of sulphur and selenium, and the balance iron with not more than 1.5% of incidental impurities.

6. As an article of manufacture for use at elevated temperatures of up to 1700 F., an alloy consisting of 0.55% to 0.65% carbon, 7.5% to 10.0% manganese, 0.45% to 0.60% silicon, 21.0% to 22.5% chromium, 0.30% to 0.40% nitrogen, and the balance iron with not more than 1.5% of incidental impurities, the alloy being formed to the predetermined shape of the article and being in the solution treated condition resulting from quenching the alloy from a temperature in the range of 1900 F. to 2300 F., the alloy anticle being further characterized by being readily weldable and being re- 15 sistant to oxidation and corrosion when exposed to the elevated temperatures.

7. An age hardened article of manufacture for use at elevated temperatures of up to 1700 F., consisting of an alloy of 0.55% to 0.65% carbon, 7.5% to 10.0% manganese, 0.45% to 0.60% silicon, 21.0% to 22.5% chromium, 0.30% to 0.40% nitrogen, and the balance iron with not more than 1.5% of incidental impurities formed to the predetermined shape of the article, the alloy article being in the age hardened condition resulting from quenching the alloy from a temperature in the 16 range of 1900 F. to 2300" 'F. and aging the alloy at a temperature in the range of 1200 F. to 1600 F. for a time period ranging between 15 minutes and 48 hours, the alloy article being characterized by its resistance to oxidation and corrosion when exposed to the elevated temperatures.

References Cited in the file of this patent UNITED STATES PATENTS 2,698,785 Jennings Jan. 4, 1955 

1. AN AGE HARDENABLE AUSTENITIC ALLOY CONSISTING OF, FROM 0.55% TO 0.65% CARBON, FROM, 7.5% TO 10.0% MANGANESE, 0.45% TO 0.60% SILICON, FROM 21.0% TO 22.5% CHROMIUM, FROM 0.30% TO 0.40% NITROGEN, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES. 